Machines such as wheel loaders, excavators, dozers, motor graders, and other types of heavy equipment use multiple actuators supplied with hydraulic fluid from one or more pumps on the machine to accomplish a variety of tasks. These actuators are typically velocity controlled based on an actuation position of an operator interface device. However, when the movement of one of the actuators is restricted by an external load, the restricted actuator can slow dramatically or even stop moving altogether even though the operator interface device is still displaced toward an actuated position (i.e., the restricted actuator can stall). If pressurized fluid continues to be allocated to the stalled cylinder based on the displacement position of the operator interface device, efficiency of the machine can be reduced. In addition, fluid pressure of the entire system can rise abruptly when any one of the machine's actuators has its movement restricted. In some situations, the rise in pressure can be high enough to cause the pump to stall and/or reduce controllability of other connected actuators. Further, because the pressure of the fluid supplied to all of the actuators is generally controlled by the single highest pressure of any one actuator in the system, during a single-actuator stall condition when system pressures rise, the flow rate of fluid supplied to all of the actuators could be needlessly reduced resulting in a general loss of production and controllability.
One method of improving machine operations during a stall condition is described in U.S. Pat. No. 7,260,931 (the '931 patent) issued to Egelja et al. on Aug. 28, 2007. Specifically, the '931 patent describes a hydraulic system for use in an excavation machine. The hydraulic system includes a first circuit supplied with pressurized fluid from a first pump and having, among other actuators, a boom cylinder. The hydraulic system also includes a second circuit supplied with pressurized fluid from a second pump and having, among other actuators, a swing motor. During a swinging movement of the excavation machine, when linkage of the machine contacts an obstacle and the swing motor is restricted from moving, fluid pressure supplied to all actuators of the second circuit rapidly increases. In response to the rapidly increasing pressure, the second pump quickly destrokes in an attempt to reduce the pressures in the second circuit and avoid stall conditions. In order to enhance controllability over movement of other actuators within the second circuit during the reducing pump output, the flow rates commanded of the second circuit actuators are scaled down according to a ratio of sensed pressure-to-stall pressure of the second pump. At this same time, any flow from the second circuit that exceeds the scaled down flow rate is diverted into the first circuit and made available to boost movement of the boom cylinder.
Although the system of the '931 patent may help to improve some machine operations during a stall condition, the system may lack applicability. In particular, the system may lack applicability to a machine having only a single circuit with a single pump, and/or to conditions associated with stall of only a subset of actuators within a single circuit.
The disclosed hydraulic control system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.